In the conversion of hides into leather an initial step is the separation of hair and epidermis from the skin or dermis, which ultimately becomes the leather. Under buffered alkaline conditions imposed by hydrated lime, dissolved sodium sulfide preferentially breaks down the hair and epidermal proteins without adversely affecting the collagens or skin protein. After the degradation process, the hair, along with the lime/sulfide liquor, is removed from the hides for disposal by washing methods.
The alkaline depilatory liquor normally contains a considerable excess of lime and has a pH of at least 12.5. The lime performs multiple functions in that the alkalinity causes the fiber bundles in the hide matrix to swell and open up, and catalyzes the depilatory reaction between the protein and the sulfides.
The disposal of the spent unhairing and liming liquor has been an ongoing problem for tanneries. It has been recognized that if the pH of the liquor is reduced much below 10.0, hydrogen sulfide gas will be stripped or released from the liquor. The development of hydrogen sulfide gas has the potential to result in increased toxicity, corrosivity and obnoxious odors. Further, a decrease in pH of the liquor by virtue of mixing with acidic components in the sewer may give rise to the emission of hydrogen sulfide, which in turn poses a potential health hazard to sewer workers and may accelerate the deterioration of sewer lines and sewer treatment equipment.
Various methods have been attempted in the past to remove sulfides from tannery waste liquor. One of the oldest techniques is chemical treatment by ferrous sulfate, which results in the precipitation of iron sulfide and iron hydroxide. The precipitation of iron hydroxide also causes a lowering of the pH and a partial precipitation of protein present in the liquor. This process has several major disadvantages, including the high volume of sludge produced for the sulfide is merely precipitated as a sludge and is not converted to a less hazardous form, or recaptured for reuse.
Catalytic oxidation has also been employed to remove sulfides from tannery waste liquor. This method consists of injecting air into the spent liquor in the presence of a suitable catalyst, such as manganese sulfate in the ratio of 1 part manganese to 10 to 20 parts sulfide. The sodium sulfide present in the liquor is oxidized primarily to thiosulfate and in smaller quantities to sodium sulfate. While the catalytic oxidation process is effective in removing sulfide, it requires the use of a heavy metal catalyst with a potentially negative environmental impact.
A further method used in the past for removing sulfides is an air stripping system, whereby the liquor is initially fed to a clarifier which is an essential part of the recovery system and serves to remove larger solids, that might interfere with operation of the de-gasifier tower. After passing through the clarifier the liquor is pumped to the degasifier tower wherein the liquor is acidified and air is passed in countercurrent flow to the liquor to air strip the hydrogen sulfide gas released from the liquor. The typical degasifier tower consists of a series of vertically spaced trays, and the liquor is pumped into the top tray and flows downward, while air is bubbled up through each tray and exits from the top of the degasifier tower. In addition, the typical stripping system includes an overflow tower which is connected to the de-gasifier tower and serves to maintain pressure in the degasifier tower. From the degasifier tower, the hydrogen sulfide gas is passed to an absorption tower where the gas passes in countercurrent flow to a sodium hydroxide solution which reacts with the hydrogen sulfide to form sodium sulfhydrate and/or sodium sulfide. As the typical stripping system requires a clarifier, a degasifier tower, an overflow tower, and an absorption tower, as well as a pumping mechanism, the capital cost of the typical stripping system is substantial.
Further, the typical air stripping systems, as used in the past, have had distinct shortcomings. More particularly, the degasifier trays in the degasifier tower become clogged with proteins that precipitate during acidification of the liquor causing substantial downtime for cleaning. In addition, control of the air flow through the degasifying tower is critical to ensure that the hydrogen sulfide concentration enroute to the absorbing tower remains below the explosive limit of hydrogen sulfide in air.
Air stripping has the further disadvantage of removing other volatiles from the unhairing liquors. These impurities have been reported to impede the depilatory effectiveness of the recovered liquors. Some oxidation of the sulfides may also take place.